Currently, an optical communication network enabling large capacity data communication at high speed such as optical communication, optical network, and optical interconnection is expanding. When performing the data communication between substrates in an electronic device with the optical communication network, the communication is performed through the following method.
Specifically, a plurality of substrates in the electronic device is connected by an optical wiring module. The data communication is performed among the substrates by transmitting the optical signal of 1 GHz through the optical wiring module among the plurality of substrates.
In recent years, a method of using a surface emitting semiconductor laser for a light source when forming the optical communication network through such a method is given attention.
The surface emitting semiconductor laser is called a VCSEL (Vertical Cavity Surface Emitting Laser), and is a laser beam emitted perpendicular to the semiconductor wafer. The surface emitting semiconductor laser can obtain high optical output by low power consumption and easily create a one-dimensional array or a two-dimensional array compared to an end face emitting type (laser beam emitted parallel to the semiconductor wafer) that has been conventionally used, and hence has various advantages such as applicability to the multi-channel optical communication network are obtained. Thus, the surface emitting semiconductor laser is expected to be suitably used as an optical fiber, optical LAN (Local Area Network), multi-channel optical interconnection, and two-dimensional parallel light calculation light source.
As described above, the optical communication network can be applied to various types of fields, and is used under various conditions. Thus, the surface emitting semiconductor laser is desired to be satisfactorily driven under any condition. To this end, improvement of the surface emitting semiconductor laser itself is obviously necessary, but at the same time, a method of driving the surface emitting semiconductor laser also needs to be sufficiently considered.
The method of driving a general surface emitting semiconductor laser will now be briefly described with reference to FIGS. 21(a) and 21(b).
FIG. 21(a) is a view showing a relationship between the power of output light of a surface emitting semiconductor laser on a transmission side and a current value.
As shown in FIG. 21(a), the surface emitting semiconductor laser starts an oscillation operation when exceeding a threshold current. When starting the oscillation operation, the surface emitting semiconductor laser performs direct modulation using a bias current (a current smaller than or equal to a current value necessary for outputting a signal of “0”) and a drive current produced when modulating the optical signal at the time of the oscillation (a modulation current and a current greater than or equal to the current value necessary for outputting the signal of “0” and smaller than or equal to the current value necessary for outputting a signal of “1”). In other words, a binary signal of “1” and “0” is generated by switching the bias current and the drive current. The binary signal generated by the direct modulation is transmitted to a reception side.
FIG. 21(b) is a view showing transmission characteristics of the surface emitting semiconductor laser on the reception side.
Specifically, FIG. 21(b) shows a so-called “eye pattern”, or a state of rise and fall of the binary signal. The eye pattern is a diagram in which reception characteristics in a case where the reception side receives the signal of “0” and reception characteristics in a case where the reception side receives the signal of “1” are overlapped. As shown in FIG. 21(b), when an opening rate of the eye pattern is relatively large, the time (oscillation delay time) until outputting an electrical signal as an optical signal is short on the transmission side, and thus it can be said that a satisfactory transmission characteristic can be obtained on the reception side. On the contrary, when the oscillation delay time becomes long, the opening rate of the eye pattern becomes small. This means that the signal quality is disturbed on the reception side. The opening rate of the eye pattern refers to the portion surrounded as if an eye by two waveforms showing the reception characteristics when the reception side receives the signal of “0” and the reception characteristics when the reception side receives the signal of “1” in the eye pattern, and a rough specification of each standard is defined by a hexagonal pattern (called eye mask).
In the surface emitting semiconductor laser, the relationship between the bias current and the threshold current greatly influences dynamic characteristics. Normally, the extinction ratio (ratio of power of output light in ON state and power of output light in OFF state) of the output light serving as the optical signal becomes smaller and the characteristics degrade as the bias current becomes larger than the threshold current. On the contrary, the oscillation delay time becomes longer the smaller the bias current compared to the threshold current. That is, the transmission characteristics on the reception side are disturbed and drawbacks occur in the data communication.
In addition, the threshold current has a very strong dependency with respect to temperature, and the threshold current greatly differs depending on the temperature (drive temperature) for driving the surface emitting semiconductor laser even when the same output light power is obtained.
Therefore, in order to satisfactorily drive the surface emitting semiconductor laser regardless of the change in the drive temperature, the relationship of the bias current, the threshold current, and the drive current needs to be sufficiently and appropriately set.
A known example of the drive method that takes into consideration the relationship between the bias current, the threshold current, and the drive current will be briefly described, where two methods will be described.
In the first method, the bias current holds the current value constant by the APC (Automatic Power Circuit) circuit, as in the invention disclosed in Patent Document 1. The temperature characteristics of the drive current are represented by synthesizing a plurality of linear characteristics, and the characteristics of the drive current itself are approximated by exp function. In the second method, one or both of the bias current and the drive current are driven by two current sources, as in the invention disclosed in Patent Document 2. The temperature characteristics and the current characteristics are approximated by the exp function. Through such methods, the threshold current and the slope efficiency (slope of the fluctuation of the power of the output light with respect to the fluctuation of the current value when the current value is raised from the bias current to the drive current) can be expressed with the exp function. The power of the output light becomes constant irrespective of the change in the drive temperature by controlling the current values in correspondence to the drive temperature.
However, the inventions disclosed in Patent Documents 1 and 2 have a problem in that application cannot be made when having the temperature characteristics in which the threshold current cannot be approximated with the exp function. Furthermore, when the temperature characteristics of the threshold current and the temperature characteristics of the slope efficiency greatly differ, the power of the output light may become more unstable.
As a method of solving the above problem, Patent Document 3 discloses an invention of a method of driving the semiconductor laser in an aim of reducing the drive current while ensuring a desired value for the extinction ratio and the oscillation delay time.
In the invention disclosed in Patent Document 3, the following conditions [A] to [C] are further set.
Condition [A]: Maintain the temperature to lower than or equal to a predetermined value in drive range (drive temperature range)
Condition [B]: Have extinction ratio constant at a lower limit of drive temperature range
Condition [C]: Have oscillation delay time constant at an upper limit of drive temperature range
Specifically, mathematical formula (A) defining an upper limit of an active layer temperature determined by a condition such as reliability of a laser is set for condition [A]:Toh+ΔT(Id)<TH  (A)
The mathematical formula (B) defining the condition of constant extinction ratio at the lower limit temperature in the drive temperature range is set for condition [B]:[Ib+Id−Ith(TL)]/[Ib−Ith(TL)]≧R  (B)and
The mathematical formula (C) defining the condition of constant oscillation delay time at the upper limit temperature in the drive temperature range is set for condition [C]:Td≦T0  (C)
(Where Toh: upper limit of ambient temperature; ΔT(Id): temperature rise of active layer caused by flowing drive current Id to semiconductor laser, TH: upper limit to be satisfied by temperature of active layer determined by conditions such as reliability, R: desired extinction ratio, Ib: bias current, Id: drive current, Ith(TL): threshold current at lower limit temperature of applicable temperature range, Td: oscillation delay time, T0: desired oscillation delay time).
The current values of the bias current and the drive current are fixed to current values that satisfy all of the above conditions [A] to [C].
Through such a drive method, in Patent Document 3, the drive current is reduced while ensuring the value desired by the optical communication network with respect to the extinction ratio and the oscillation delay time, and the power of the output light is made constant.    Patent Document 1: Japanese Unexamined Patent Publication No. 3-133187 (date of publication: Jun. 6, 1991)    Patent Document 2: Japanese Unexamined Patent Publication No. 10-209538 (date of publication: Aug. 7, 1998)    Patent Document 3: Japanese Unexamined Patent Publication No. 5-190947 (date of publication: Jul. 30, 1993)